1. Field
The present work relates generally to wireless communications and, more particularly, to contention for access to portions of a wireless communication spectrum.
2. Background
The four documents identified below are incorporated herein by reference. They are referred to herein by the corresponding bracketed numerals below.                [1] Federal Communications Commission Spectrum Policy Task Force, ‘Report of the Spectrum Efficiency Group’, Technical Report 02-135.        [2] Federal Communication Commission, Second Report and Order on Unlicensed Operation in the TV Broadcast Bands, November 2008, FCC 08-260.        [3] Shared Spectrum Company, “Comprehensive Spectrum Occupancy Measurements Over Six Different Locations”, August 2005.        [4] Draft Standard for Wireless Regional Area Networks Part 22, IEEE P802.22™/DRAFTv2.0., May 2009        
One of the major limitations facing the continuous increase in demands for higher data rate services and future wireless applications is the scarcity of spectrum. On the other hand measurements studies conducted by the FCC and industry [1, 2] show light utilization of the spectrum at any given time or location. This is especially true for the TV spectrum, a high percentage of which is under utilized, or what is referred to as the White Space.
One solution to increase the efficiency of utilizing this spectrum and provide a solution for the spectrum scarcity is opportunistic usage of the under utilized spectrum without causing interference to the spectrum primary users or incumbents. In order to encourage unlicensed use of the spectrum, the FCC, through a Notice of Proposed Rule Making (FCC NPRM) is considering opening up the spectrum allocated to TV broadcast services for unlicensed use by secondary devices operating on a non-interference basis with the incumbent users (TV broadcasters). The IEEE has also set up a working group (802.22 WG), which is drafting an emerging standard for opportunistic broadband data communication over Wireless Regional Area Networks (WRANs) utilizing cognitive radio technologies in the TV frequency spectrum [3]. The term ‘cognitive radio’ can be thought of as encompassing several techniques that seek to overcome the spectral shortage problem by enabling secondary (unlicensed) wireless devices to communicate without interfering with the primary users. Cognitive communication faces a multitude of hurdles in spectrum access and spectrum sharing. A spectrum sharing protocol is a technique to enable reliable operation of two or more systems sharing the same spectrum. Therefore a spectrum sharing protocol is essentially an interference management scheme between different systems.
Spectrum sharing techniques fall into different categories based on the level of information and cooperation between the different secondary systems. In particular, spectrum sharing can be categorized into three broad approaches:                (1) The networks operate independently trying to access the medium, or in other words use non-cooperative approaches;        (2) The networks agree on a set of rules that do not require interoperability, for example listen-before-talk; and        (3) A common message approach in which different systems can utilize a common control channel to exchange information related to sharing the spectrum, which requires a level of interoperability between systems.        
The IEEE 802.22 [4] system specifies a fixed wireless regional area network (WRAN) in which a base station (BS) manages a group of fixed customer premise equipments (CPEs) as depicted in FIG. 1. The IEEE 802.22 draft standard [4] describes a message based approach for self-coexistence among 802.22 WRANs. The draft standard defines a Coexistence Beacon Protocol (CBP) in which the CPEs and BSs are allowed to transmit coexistence information during a predefined coexistence window, namely a self-coexistence window (SCW), which is a specified duration of time in the UL (uplink) sub-frame. This is illustrated by the timing diagram of FIG. 2, wherein TTG is the Transmit/Receive Transition Guard, and RTG is the Receive/Transmit Transition Guard. The coexistence beacons are transmitted in the SCW either by the BS or the CPE and can carry information about the bandwidth allocations of the cell, and hence neighboring cells can implement interference-free scheduling. The BS decides whether the coexistence beacon is transmitted by the BS or one or more of its associated CPEs. The CBP can also carry timestamps for synchronizing BSs, which is required for coexistence.
The CBP enables the WRANs to perform an on demand contention protocol for spectrum sharing [4]. In this protocol, the contention destination (CD) is defined as the WRAN BS currently occupying the channel. The contention sources (CS) are defined as all WRAN BSs that wish to contend to access this channel. In order to decide on the winning CS, the following steps are implemented, as shown in FIG. 3.
Each CS randomly generates a contention number (CN) as shown at 31. The CS's try transmitting their CN's during the SCW (self-coexistence-window) through contention with other CS's, as shown collectively at 32-38. After all CN's are received at the CD, the CD decides that the winning CS is the CS with the largest CN.
The on demand contention protocol of FIG. 3 has disadvantages, some of which include: (1) All CS's have to complete the transmission of the generated CN's and then the CD chooses the CS with the highest CN; (2) There is a large probability of collision due to the probability of different CS's using the same size random back-off window; (3) If there are M CS's, then at least M SCW frames are required to complete the process (typically the number of SCW frames is larger than M, and the algorithm therefore does not scale well); (4) If the winning CS only needs to transmit x frames with x small, then at least M−x frames are wasted until that winning CS acquires the channel.
It is desirable in view of the foregoing to provide for spectrum contention procedures capable of avoiding prior art disadvantages such as described above.